(Not Applicable)
Synthetic aperture radar (SAR) is used to obtain two-dimensional images of the ground from an airborne platform. The SAR antenna typically points toward the ground perpendicular to the path of the aircraft. SAR collects data while flying a predictable path (either a straight line to view a strip of ground or a circular path to view a spot of ground) and then processes the data as if it came from a physically long antenna. The distance the aircraft flies in synthesizing the antenna is known as the synthetic aperture. A narrow synthetic beamwidth results from the relatively long synthetic aperture, which yields finer resolution than is possible from a smaller physical antenna. SAR systems may survey the ground from an aircraft several miles from the target, and they may provide two dimensional views of the target with a resolution better than one foot.
Interferometric synthetic aperture radar (IFSAR) is a relative of SAR which uses two antennas separated in elevation to produce estimates of terrain elevation in a SAR image; i.e., a three dimensional view of the ground. Each transmitted pulse is received at the two antennas, and phase difference information from the return pulse at each antenna is used to estimate the angle of arrival of the received pulse. For a level radar platform, changes in angle-of-arrival indicate a change in elevation of the target. Detailed background information on IFSAR may be found in D. Bickel et al., xe2x80x9cDesign, Theory, and Applications of Interferometric Synthetic Aperture Radar for Topographic Mapping,xe2x80x9d Sandia National Laboratories Report SAND96-1092, May 1996, available through DOE/OSTI.
Since the path of the aircraft may deviate from an ideal straight line by considerably more than one foot, high quality SAR and IFSAR systems utilize a combination of state of the art electronics, algorithms, data processing, and hardware to compensate for spurious motion of the vehicle, the effects of weather on the signal, and many other factors. Detail is important, as anything that is overlooked is likely to degrade the image.
High quality prior art IFSAR systems have experienced difficulty controlling the phase response of the system as a function of elevation angle. This lack of control often puts xe2x80x98wavesxe2x80x99 of height error into the elevation output of the system. This invention is the result of the inventors"" understanding that the cause of these waves is low-amplitude signals reflected from metal near the antennas (such as the body of the aircraft) that add to the direct-path signal. These multi-path signals cause phase errors that produce an unpredictable system phase response as a function of elevation angle.
It is an object of this invention to significantly reduce the amplitude of reflected signals from the aircraft which add to the direct-path signal.
It is another object of this invention to provide a conical diffraction grating around the radar to direct reflected signals from the aircraft away from impinging the radar antenna.
It is also an object of this invention to provide a shroud around the radar antennas to prevent undesirable signals from impinging the radar antennas.
To achieve the foregoing and other objects, and in accordance with the purpose of the present invention, as embodied and broadly described herein, a system for reducing phase errors from scattering in an IFSAR comprises a shroud made of radar reflective material extending around the IFSAR antenna surface, the shroud shielding said surface from reflected energy from objects adjacent said surface and not along said beam. Furthermore, the invention comprises a radar reflective cone mounted to the aircraft and above the IFSAR to reflect energy which strikes the aircraft adjacent the IFSAR away from the IFSAR.
Additional objects, advantages, and novel features of the invention will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.